FIG. 1A represents a maritime scenario 10 in which the horizon is designated 12, and two friendly Aegis ships 14 and 16 communicate by a path illustrated by “lightning bolt” symbol 18. An object or “target” illustrated as 20 is near ship 14, and is detected by sensors (not separately illustrated) aboard ship 14, as suggested by lightning bolt symbol 22. Ship 16 has not sensed the presence of object 20. Target location information in ship 14 coordinates (cue information) is transmitted by way of communication path 18 from ship 14 to ship 16. The purpose of the cue information is to enable ship 16 to direct its radar sensor toward the target without the need to search the entire environment. FIG. 1B is a simplified block diagram of equipment or functions on ship 16. In FIG. 1B, the SPY radar system 40 communicates with a command and control function 42. Command and control function 42 receives cue information in the form of target state (location) and covariance (error) and converts the cue information to local (ship 16) coordinates. In addition, command and decision 42 determines from the variance or error the extent of the region that must be searched to be sure of locating the target.
SPY radar systems are used on AEGIS warships. The SPY radar systems use sequential pencil beams to search an acquisition face. These warships or platforms are equipped with communications facilities which allow sharing of sensor information among various spaced-apart platforms. In one version, the command and decision portion of the radar system receives, from a remote source, cue information relating to the approximate location of a target(s), which approximate location is defined by a state and associated covariance or uncertainty. The command and decision logic calculates the size of the necessary search region (acquisition face and range), and sends the resulting search volume to the SPY radar control.
In theory, SPY radar control places a pencil beam at the center of the search face specified by command and control, and determines the center location of the beams vertically and horizontally adjacent to the center beam. The center locations of the current beam are compared with the acquisition boundaries (the boundaries of the acquisition face), and if it lies within the boundaries the beam is scheduled for use. If the beam center lies without the acquisition boundaries, it is dropped, and not scheduled. When all the beams have been scheduled for a given search, the beams are sequentially generated, starting at the center of the search face, and expanding the search by placing beams in a “rosette” pattern about the center in an expanding manner. This allows the most likely position of the target to be searched first.
FIG. 2A represents a search face 212 as defined by the command and decision portion of a warship in response to cue information received from a remote source. In FIG. 2A, the search face 212 is defined by horizontal and vertical lines 214, 216, 218, and 220. The center of the pattern is identified by a cross or plus (+) symbol. The elevation extent of the search face 212 as defined by the command and decision equipment or function extends from the center (+) to horizontal line 214 or 216 and is designated “El extent”. The azimuth extent of the search face as defined by command and decision extends from the center (+) to vertical line 218 or 220. Circle 230 of FIG. 2A represents the first pencil beam, which is ultimately placed at the center of the search face. The rosette pattern by which placement of further beams is controlled places the second beam 232 on one side of central beam 230, with its center separated in azimuth from the center of central beam 230. The third beam 234 is place on the other side of the central beam 230, similarly displaced in azimuth. FIG. 2B illustrates a central rosette 233 of six beams. In FIG. 2B, the rosette includes central beam 230, first and second azimuthally-displaced beams 232 and 234, and additional beams 236, 238, 240, and 242, which can be added in the sequence defined by their reference numerals. This pattern continues, with the addition of more beams, until the search face 212 of FIG. 2A is filled. It should be emphasized that actual beam placement does not begin until all the calculations are finished and all the beams are scheduled.
In operation of the basic SPY system, the beams are added in the rosette pattern so long as the beam centers lie within the area defined by lines 214, 216, 218, and 220. FIG. 2C illustrates some beams 260, 261, and 262 lying near edge 216. As illustrated, these beams at the boundaries of the search face overlap, as illustrated in FIG. 2C. In FIG. 2C, beams 260, 261, and 262 have their centers within the boundary of the acquisition face 212 of FIG. 2A. As illustrated, the overlap of beams 260, 261, and 262 leaves an approximately triangular “scallop” region unfilled between each beam. More particularly, the scallop region between beams 260 and 261 is designated 260/261s, and the scallop region between beams 260 and 262 is designated 260/262s. These scallop regions are only nominally unfilled, since the illustrated beam contour actually represents a region in which beam energy exists, but at a lower magnitude or intensity than the design values. One possible way to go forward is to ignore the slight loss of coverage occasioned by scalloping.
In order to fill the scallop regions, the SPY radar system modifies the boundaries 214, 216, 218, and 220 illustrated in FIG. 2A (as specified by command and decision) by adding vertical and horizontal “pads.” FIG. 2D illustrates the search face or pattern of FIG. 2A with the addition of padding in the form of extended boundaries. The boundaries are extended in the horizontal direction by an amount delta azimuth (ΔA) and in the elevation or vertical direction by an amount delta E (ΔE). The resulting extended boundaries are designated in FIG. 2D as 214e, 216e, 218e, and 220e. The magnitudes of ΔA and ΔE are selected to be equal to the spacing between the centers of the beams. The SPY radar system then fills in the extended boundaries 260 with beams following the same “rosette” pattern previously described, again so long as the beam centers lie within the extended boundaries. The additional beams are scheduled if the beam centers lie within the extended boundaries and are dropped if they lie without. Once all the beams have been processed, those scheduled can be generated.
Thus, the processing in the SPY radar block 40 of FIG. 1A may be represented by the logic or control logic 300 of FIG. 3. In FIG. 3, logic 300 begins at a START block 310, and flows to a block 312. Block 312 represents the receipt or acquisition of information relating to the center angle of the search face, and the angular extents in azimuth and elevation, as determined by Command & Decision block 42 of FIG. 1B. Block 314 represents the addition of azimuth and elevation padding. This padding is a predetermined angle for each of azimuth and elevation. The azimuth and elevation padding may be in the same amount. The logic 300 of FIG. 3 flows to a block 316, which represents scheduling the “placement” of an antenna beam at the center of the reported or cued search face. It should be understood that the placement may be only a scheduling, rather than actual formation of the beam, with the actual beam placement or generation delayed until all the calculations are complete. From block 316, the logic 300 flows to a block 318, which represents the calculation of the location of the center of the next beam of the “rosette” pattern. A decision block 320 determines whether or not the beam center location so calculated lies within the padded extent. If the beam center does not lie within the padded extent, the logic leaves decision block 320 by the NO output, and arrives at a block 324. Block 324 represents the non-scheduling or ignoring of the beam. On the other hand, if decision block 320 finds that the beam center lies within the padded extent, the logic flows to a block 322, which represents the scheduling of the beam. From either block 322 or 324, the logic flows to a further decision block 326, which determines if all beams have been evaluated. If not, the logic leaves decision block 326 by the NO output, and returns by a logic path 328 to block 318, to begin another iteration through the loop including blocks 318, 320, 322, 324, and 326. Eventually, all the beams will have been evaluated, and the logic leaves decision block 326 by the YES output, and arrives at a logic END block 330.
FIG. 4 is a simplified logic or control flow chart or diagram 400 illustrating relevant portions of the operation of Command & Decision block 42 of FIG. 1B. In FIG. 4, the logic begins at a block 412, which represents the receipt of cue data from a remote source. The remote source may be, for example, a forwardly-located or picket ship, such as ship 14 of FIG. 1. From block 412, the logic 400 flows to a block 414, which represents calculation of the acquisition volume. The cue information about the location of the target is converted to local coordinates, and the acquisition face area is determined by Command and Decision by considering the nominal state or location of the target 20 as reported, and expanding the region depending upon the covariance or error in the measured location. The acquisition volume is determined from the acquisition face area and the reported range of the target, with its covariance. From block 414, the logic 400 flows to a block 416.
Block 416 of FIG. 4 represents determination of the number of beams required to cover the padded search face, which is illustrated as 212 in FIG. 1A. This determination of the number NE of beams is made by equation (1)
      N    E    =                    [                  1          +                      2            ⁢                          floor              ⁡                              (                                                                            A                      ext                                                              2                      ⁢                      Δ                      ⁢                                                                                          ⁢                      A                                                        +                                      1                    2                                                  )                                                    ]            ⁡              [                  1          +                      2            ⁢                          floor              ⁡                              (                                                                            E                      ext                                                              2                      ⁢                      Δ                      ⁢                                                                                          ⁢                      E                                                        +                                      1                    2                                                  )                                                    ]              +                  4        ⁡                  [                      1            +                          floor              ⁡                              (                                                      A                    ext                                                        2                    ⁢                    Δ                    ⁢                                                                                  ⁢                    A                                                  )                                              ]                    ⁡              [                  1          +                      floor            ⁡                          (                                                E                  ext                                                  2                  ⁢                  Δ                  ⁢                                                                          ⁢                  E                                            )                                      ]            whereNE is the number of beams SPY must use to completely cover the extended (padded) search face.Aext is the azimuth extent (Az extent in FIG. 2A),Eext is the elevation extent (El extent in FIG. 2A),ΔA and ΔE are delta A and delta E as defined above,floor(x) is a function that truncates its argument x into the highest integer less than or equal to x.From block 416, the logic 400 of FIG. 4 flows to a block 418.
Block 418 of FIG. 4 represents calculation of the time required to search the acquisition volume. This time is nominally the number of beams in the acquisition face multiplied by the time for the energy of a beam to reach the maximum range and to return. This time is the well-known 10.8 microseconds (μS) per statute mile or 12.4 microseconds per nautical mile. Decision block 420 compares the time required to search the nominal acquisition volume with the maximum time allowable per search by the SPY radar system (block 40 of FIG. 1B). If the time required to search the nominal acquisition volume is less than the maximum, logic 400 of FIG. 4 leaves decision block 420 by the YES output, and flows to a block 426. Block 426 represents the sending of the acquisition volume from Command & Decision 42 to SPY radar 40 for generation of the beams to cover the search area as described in conjunction with FIGS. 2A, 2B, 2C, and 2D.
If the time required to search the acquisition volume is determined in decision block 420 to be greater than the allowable search time, the logic 400 of FIG. 4 leaves the decision block by the NO output, and flows to a block 422. Block 422 represents partition of the search volume, and the parameters of the partitioned acquisition volumes are returned by a logic path 424 to block 414 to start another iteration through the logic 400.